1. Field of the Invention
The present invention relates to an electrostatic chuck for electrostatically attracting and holding a workpiece such as a semiconductor wafer or the like.
2. Description of Relevant Art
One conventional electrostatic chuck plate disclosed in Japanese patent publication No. 60-59104, which corresponds to U.S. Pat. No. 4,480,284, has a dielectric layer formed on an electrode plate for securely holding a workpiece such as a semiconductor wafer thereon under an electric field developed between the electode plate and the semiconductor wafer while the semiconductor wafer is being processed by plastma etching, CVD, ion plating, or the like in a vacuum. When a voltage is applied between the electrode plate and the workpiece, the workpiece is attracted and held on the electrostatic chuck plate under large electrostatic attractive forces. When the voltage applied between the elecrode plate and the workpiece is removed, any electrostatic attractive forces that have been developed are immediately reduced, allowing the workpiece to be easily released from the electrostatic chuck plate.
To increase attractive forces produced by an electrostatic chuck, the dielectric constant of the dielectric layer thereof may be increased as disclosed in Japanese patent publications Nos. 60-59104 and 62-19060, or the thickness of the dielectric layer thereof may be controlled as disclosed in Japanese laid-open patent publication No. 57-64950 corresponding to U.S. Pat. No. 4,384,918, or the volume resistivity of the dielectric layer thereof may be selected in a predetermined range as disclosed in Japanese patent publication No. 61-14660. To facilitate removal of the workpiece from the electrostatic chuck, a helium gas may be introduced between the workpiece and a chuck surface to which the workpiece is attracted as disclosed in Japanese laid-open utility model publication No. 2-120831, or a voltage of opposite polarity to the polarity of the voltage applied to attract the workpiece is applied as disclosed in Japanese patent publication No. 2-63304.
The various conventional schemes for increasing the attractive forces are primarily concerned with the dielectric layer. Therefore, the greater the attractive forces, the greater the residual attractive forces that remain present after the applied voltage is removed. It takes 60 seconds or longer until the residual attractive forces are reduced by 98% or more allowing the workpiece to be easily detached from the conventional electrostatic chuck. Thus, the workpiece cannot immediately be removed after it has been processed while being held by the electrostatic chuck. For easy and immediate removal of the workpiece from the electrostatic chuck, therefore, it is necessary to employ another removal device in addition to the electrostatic chuck or a new removal process in addition to the ordinary process combined with the electrostatic chuck. The additional device or process for easy and immediate workpiece removal however poses problems on its use at low temperatures.
Recent years have seen the problem of contamination of semiconductor wafers with metal, resulting in a reduced semiconductor device yield. One solution has been to deposit a protective film on the attractive surface of an electrostatic chuck for protecting semiconductor wafers against contamination from the electrostatic chuck. Depending on the electric characteristics of such a protective film, the electrostatic chuck may not develop sufficient attractive forces for holding semiconductor wafers.
Japanese laid-open patent publication No. 2-22166, which corresponds to U.S. Pat. No. 5,104,834, assigned to the same assignee of the present application, discloses a dielectric ceramic material for an electrostatic chuck and a method of manufacturing such a dielectric ceramic material.